Is it possible for light sources in Cycles to have nonphysical falloff like in BI?
Yes. There is a node called Light Falloff
you can plug in to the Emission shader strength.
It has three outputs: quadratic or Linear
or Constant You my even do some math with the ray
length and plug the result in the Light Falloff
node strength and use the Constant output.
The example is only a demonstration nothing useful.